Excited-state properties of the central-cis isomer of the carotenoid peridinin

Abstract

The central-cis isomer of the carotenoid peridinin, presumably 13-cis, was separated and studied with spectroscopic methods including static absorption, fluorescence and femtosecond time-resolved absorption. The investigations exposed differences in the photophysical properties of this isomer in respect to all-trans peridinin. Steady-state absorption spectroscopy revealed the presence of an additional weak absorption band at the long wavelength tail of the main S0 → S2 transition. Modelling of the hypothetical vibronic progression of the S0 → S1 electronic transition demonstrated that this weak band can be associated with a higher (0–2) vibronic band of the transition and that lower vibronic bands have negligible intensities due to a large displacement between the S0 and S1 states energy curves as also suggested by the spectral shape of steady-state fluorescence emission. Transient absorption studies demonstrated that the lifetime of the S1 state of the central-cis isomer is shorter compared to the all-trans counterpart by 6–16%, depending on the polarity of the solvent. On the other hand, molecular isomerization negligibly affects the lifetime of intramolecular charge transfer (ICT), which for both isomers is ∼10 ps in the polar solvent methanol.